1. Field of the Invention
The present invention relates to a zoom lens system for photographic camera, and in particular, relates to a zoom lens system for a lens-shutter camera.
2. Description of the Prior Art
Unlike a zoom lens system of a single lens reflex (SLR) camera which requires space to accommodate a quick-return mirror behind the photographing lens system, a zoom lens system of a compact camera does not require a long back focal distance. As an example of such a zoom lens system of a compact camera having few constrains on the back focal distance, a zoom lens system of a three-lens-group arrangement, i.e., a positive lens group, another positive lens group, and a negative lens group, in this order from the object, has been proposed (e.g., Japanese Unexamined Patent Publication No. Hei-2-256015). However, if an attempt is made to further increase the zoom ratio in such a zoom lens system mentioned above, the overall length of the zoom lens system becomes longer at the long focal length extremity.
Furthermore, for the purpose of achieving further miniaturization and a higher zoom ratio, a zoom lens system of a four-lens-group arrangement, i.e., a positive lens group, a negative lens group, a positive lens group and a negative lens group, in this order from the object, has been proposed (e.g., Japanese Unexamined Patent Publications No. Hei-6-265788 and No. 2000-180725). However, in such a lens arrangement, the traveling distances of the lens groups thereof are longer, so that the overall length of the zoom lens system at the long focal length extremity becomes longer; and the entrance pupil position becomes distant at the short focal length extremity, so that the frontmost lens diameter becomes larger. Consequently, further miniaturization cannot be achieved.
The present invention provides a zoom lens system, for a lens-shutter compact camera with a retractable lens barrel, having a zoom ratio Z (=fT/fW) of more than 3.5.
According to the present invention, there is provided a zoom lens system including a first lens group having a positive refractive power (hereinafter, positive first lens group), a second lens group having a negative refractive power (hereinafter, negative second lens group), a third lens group having a positive refractive power (hereinafter, positive third lens group), and a fourth lens group having a negative refractive power (hereinafter, negative fourth lens group), in this order from the object.
Zooming is performed by moving each of the positive first through the negative fourth lens groups along the optical axis.
The zoom lens system satisfies the following condition:
0.35 less than (f23T/f23W)/(fT/fW) less than 0.55xe2x80x83xe2x80x83(1)
wherein
f23T designates the combined focal length of the negative second lens group and the positive third lens group at the long focal length extremity;
f23W designates the combined focal length of the negative second lens group and the positive third lens group at the short focal length extremity;
fT designates the focal length of the entire the zoom lens system at the long focal length extremity; and
fW designates the focal length of the entire the zoom lens system at the short focal length extremity.
The zoom lens system preferably satisfies the following condition:
0.05 less than (D23Wxe2x88x92D23T)/fW less than 0.2xe2x80x83xe2x80x83(2)
wherein
D23W designates the axial distance between the negative second lens group and the positive third lens group at the short focal length extremity;
D23T designates the axial distance between the negative second and the positive third lens group at the long focal length extremity; and
fW designates the focal length of the entire the zoom lens system at the short focal length extremity.
The zoom lens system can satisfy the following condition:
0.5 less than fW/f1G less than 0.7xe2x80x83xe2x80x83(3)
wherein
fW designates the focal length of the entire the zoom lens system at the short focal length extremity; and
f1G designates the focal length of the positive first lens group.
The zoom lens system preferably satisfies the following condition:
12 mm less than f4G(m4Txe2x88x92m4W)/(fT/fW) less than 14 mmxe2x80x83xe2x80x83(4)
wherein
f4G designates the focal length of the negative fourth lens group;
m4T designates the magnification of the negative fourth lens group when an object at an infinite distance is in an in-focus state at the long focal length extremity;
m4W designates the magnification of the negative fourth lens group when an object at an infinite distance is in an in-focus state at the short focal length extremity;
fT designates the focal length of the entire the zoom lens system at the long focal length extremity; and
fW designates the focal length of the entire the zoom lens system at the short focal length extremity.
The zoom lens system of the present invention can be applied to a zoom lens system in which (i) the negative second lens group and the positive third lens group are arranged to maintain a predetermined distance d1 in a short-focal-length side zooming range which is defined between the short focal length extremity and a first (before switching) intermediate focal length, and to maintain another predetermined distance d2, which is smaller than the predetermined distance d1, in a long-focal-length side zooming range which is defined between a second (after switching) intermediate focal length and the long focal length extremity; (ii) at the first (before switching) intermediate focal length, all the lens groups are moved toward an image to the second (after switching) intermediate focal length; and (iii) the zoom lens system preferably satisfies the following condition:
12 mm less than (X4W+X4Txe2x88x92xcex94X4MM*)/(fT/fW) less than 14 mmxe2x80x83xe2x80x83(5)
wherein
X4W=f4G(m4Mxe2x88x92m4W);
X4T=f4G(m4Txe2x88x92m4M*);
xcex94X4MM*=f4G(m4Txe2x88x92m4M*);
m4M=fm/f123M;
m4W=fW/f123W;
m4T=fT/f123T;
m4M*=fmxe2x80x2/f123M*;
fm designates the first intermediate focal length;
fmxe2x80x2 designates the second intermediate focal length;
f123W designates the combined focal length of the positive first lens group, the negative second lens group and the positive third lens group at the short focal length extremity;
f123M designates the combined focal length of the positive first lens group, the negative second lens group and the positive third lens group at the first (before switching) intermediate focal length in the short-focal-length side zooming range;
f123M* designates the combined focal length of the positive first lens group, the negative second lens group and the positive third lens group at the second (after switching) intermediate focal length in the long-focal-length side zooming range;
f123T designates the combined focal length of the positive first lens group, the negative second lens group and the positive third lens group at the long focal length extremity;
fT designates the focal length of the entire the zoom lens system at the long focal length extremity; and
fW designates the focal length of the entire the zoom lens system at the short focal length extremity.
In the zoom lens system, the positive third lens group preferably includes at least one aspherical surface which satisfies the following condition:
xe2x88x9230 less than xcex94IASP less than xe2x88x9210xe2x80x83xe2x80x83(6)
wherein
xcex94IASP designates the amount of change of the spherical aberration coefficient due to the aspherical surface in the positive third lens group under the condition that the focal length at the short focal length extremity is converted to 1.0.
In the zoom lens system, the negative fourth lens group preferably includes at least one aspherical surface which satisfies the following condition:
0 less than xcex94VASP less than 3xe2x80x83xe2x80x83(7)
wherein
xcex94VASP designates the amount of change of the distortion coefficient due to the aspherical surface in the negative fourth lens group under the condition that the focal length at the short focal length extremity is converted to 1.0.
The present disclosure relates to subject matter contained in Japanese Patent Application No.2002-348570 (filed on Nov. 29, 2002) which is expressly incorporated herein in its entirety.